There is water and air. If the main character is completely under water for 10 seconds, they drown. When the character touches air, they can breathe, so they will not drown 10 seconds if they go under water again.
Example: Cat touches air, has 10 seconds of breath. Cat goes completely underwater, breath goes 9..8..7… Cat touches air again, breath back up to 10. Cat goes completely under water, breath goes 9..8..7..6..5..4..3..2..1.. and then the Cat drowns (show an end scene).